Process for casting a beta-titanium alloy

ABSTRACT

The invention relates to a process for casting objects from a β-titanium alloy comprising titanium-molybdenum with a molybdenum content of from 7.5 to 25%. The invention provides for melting the alloy at a temperature of over 1770° C., investment-casting the molten alloy into a casting mold corresponding to the object to be produced, hot isostatic pressing, solution annealing and then quenching. The process according to the invention provides economic production of objects made from β-titanium alloys using the investment-casting process. The invention thereby provides the possibility of combining the advantageous properties of β-titanium alloys, in particular their excellent mechanical properties, such as the low modulus of elasticity, with the advantages of producing castings using the investment-casting process. The invention means that even objects of complex shapes, which it was impossible to produce (economically) by conventional forging processes can be produced from a β-titanium alloy.

The invention relates to a process for casting objects from a β-titanium alloy, more specifically a titanium-molybdenum alloy.

Titanium alloys are becoming more and more popular on account of their numerous advantageous properties. Titanium alloys are used in all fields in which high demands are imposed on the material, in particular on account of their good chemical stability, even at high temperature, and their low weight combined with excellent mechanical properties. On account of their excellent biocompatibility, titanium alloys are also preferentially used in the medical sector, in particular for implants and prostheses.

Various methods for shaping titanium alloys are known. In addition to cutting processes, these primarily include casting and forging processes. In principle, titanium alloys are forging alloys, for which reason forging processes are generally used, since it has been found that titanium alloys are difficult to cast. This approach is generally taken for complicated shapes but leads to restrictions in terms of the choice of suitable alloys. In particular, it has been found that only unsatisfactory results are achieved when casting β-titanium alloys (US-A 2004/0136859).

The invention is based on the object of providing an improved casting process for β-titanium alloys which allows even complex shapes to be produced with good material properties.

The solution according to the invention resides in a process having the features of the main claim. Advantageous refinements form the subject matter of the subclaims.

According to the invention, in a process for casting objects from a β-titanium alloy comprising titanium-molybdenum with a molybdenum content of from 7.5 to 25%, it is provided that the alloy is melted at a temperature of over 1770° C., the molten alloy is investment-cast into a casting mold corresponding to the object to be produced, is hot-isostatically pressed, solution-annealed and then quenched.

In the present context, an object is to be understood as meaning a product which has been shaped for final use. The object may, for example in the aeronautical industry, be parts used for jet engines, rotor bearings, wing boxes or other supporting structure parts, or in the field of medicine may be endoprostheses, such as hip prostheses, or implants, such as plates or pins or dental implants. The term object in the context of the present application does not encompass billets which are intended for further processing by shaping processes, i.e. in particular does not include ingots produced by permanent mold casting for further processing by forging.

The process according to the invention achieves economical production of objects made from β-titanium alloys using the investment-casting process. The invention therefore provides the possibility of combining the advantageous properties of β-titanium alloys, in particular their excellent mechanical properties, with the advantages of production of objects using the investment-casting process. The invention allows even objects of complex shapes, which it has been impossible to produce (economically) using conventional forging processes, to be produced from a β-titanium alloy. Therefore, the invention also opens up the application area of complex-shaped objects to β-titanium alloys, which are known to have favorable mechanical properties and biocompatibility.

The molybdenum content in the alloy or its molybdenum equivalent is in the range from 7.5 to 25%. The result of this is that, in particular for a molybdenum content of at least 10%, the β-phase is sufficiently stabilized even as far as the room temperature range. It is preferable for the content to be between 12 and 16%. This allows a meta-stable β-phase to be achieved by rapid cooling following the investment casting. There is generally no need to add further alloy-forming elements. In particular, there is no need to add vanadium or aluminum.

It has been found that the invention, using the β-titanium alloys which have hitherto been almost impossible to use for investment casting, allows the production of even more complex shapes than the α/β-titanium alloys which have hitherto been used for investment casting, such as for example TiAl6V4. The process according to the invention achieves improved mold filling properties. This means that as a result of the invention, in particular sharp edges can be produced with a higher quality during investment casting. The susceptibility to the formation of voids in investment casting is also reduced.

It is expedient for a cold-wall crucible vacuum induction installation to be used to melt the β-titanium alloy. An installation of this type makes it possible to reach the high temperatures which are required for reliable melting of titanium-molybdenum alloys for investment casting. For example, the melting point of TiMo15 is 1770° C. A supplement of approx. 60° C. should expediently be added to this to effect reliable investment casting. In particular, therefore, a temperature of 1830° C. has to be reached for TiMo15.

It is preferable for the hot isostatic pressing to take place at a temperature which is at most equal to a beta transus temperature of the titanium-molybdenum alloy and is no more than 100° C., preferably 40° C., below the beta transus temperature.

The hot isostatic pressing, in addition to conventional advantages of eliminating microporosity, also dissolves inter-dendritic precipitations. A temperature below the beta-transus temperature, specifically at most 100° C., preferably 40° C., below it is favorable. Temperatures in the range from 710° C. to 760° C., preferably of approx. 740° C., at an argon pressure of approximately 1100 to 1200 bar have proven suitable for a titanium-molybdenum alloy with a molybdenum content of 15%.

Temperatures of at least 700° C. to 900°, preferably in the range from 780° C. to 880° C., have proven suitable for solution annealing. Argon is preferably used to produce a shielding gas atmosphere. This improves the ductility of the alloy.

It is expedient for quenching of the object by water to be carried out after the solution annealing. It is preferable to use cold water. In this context, the term “cold” is to be understood as meaning the temperature of unheated tap water. It has been found that the quenching has a considerable influence on the mechanical properties of the object which are ultimately achieved. Alternatively, quenching may also take place in shielding gas, for example by argon cooling. The results achieved, however, are not as good as those achieved with cold water.

It may be expedient for the object finally also to be hardened. This may allow the modulus of elasticity to be increased slightly. For this purpose, it is preferable for the hardening to be carried out in a temperature range from approx. 600° C. to approx. 700° C.

The invention is explained in more detail below with reference to the drawing, which illustrates an advantageous exemplary embodiment. In the drawing:

FIG. 1 shows a table which gives mechanical properties of the investment-cast titanium alloy according to the invention;

FIG. 2 shows an image of the microstructure in a cast state immediately after casting;

FIG. 3 shows an image of the microstructure after hot isostatic pressing;

FIG. 4 shows an image of the microstructure after solution annealing with a subsequent quench; and

FIG. 5 illustrates liquidus and solidus temperatures for a titanium-molybdenum alloy.

The text which follows describes a way of carrying out the method according to the invention.

The starting material is a β-titanium alloy with a molybdenum content of 15% (TiMo15). This alloy can be obtained commercially in the form of billets (ingots).

A first step involves investment casting of the objects that are to be cast. A casting installation is provided for melting and casting the TiMo15. This is preferably a cold-wall crucible vacuum induction melting and casting installation. An installation of this type can reach the high temperatures which are required for reliable melting of TiMo15 for investment casting. The melting point of TiMo15 is 1770° C., plus a supplement of approx. 60° C. for reliable investment casting. Overall, therefore, a temperature of 1830° C. has to be reached. The investment casting of the melt then takes place using processes which are known per se into ceramic molds as lost molds. Investment casting techniques of this type are known for the investment casting of TiAl6V4.

As can be seen from the figure (1000 times magnification) in FIG. 2, dendrites are formed, and considerable precipitations are evident in inter-dendritic zones. This is a consequence of what is known as the negative segregation of titanium-molybdenum alloys. This effect is based on the specific profile of the liquidus and solidus temperatures of titanium-molybdenum alloys, as illustrated in FIG. 5. On account of the profile of the melting temperatures of the liquid phase (T_(L)) and the solid phase (T_(S)) illustrated, it is firstly the regions with a high molybdenum content which solidify in the melt, during which process the dendrites that can be seen in the figure are formed. This leads to depletion of the residual melt, i.e. its molybdenum content drops. The inter-dendritic zones in the cast microstructure have a molybdenum content of less than 15%, and it is even possible for the molybdenum content to drop to approx. 10%. As a result of the molybdenum depletion, the inter-dendritic zones lack a sufficient quantity of β-stabilizers. The result of this is that an increased α/β transformation temperature is locally established, resulting in the formation of the precipitations shown in FIG. 2.

It is expedient for a surface zone which may form during casting as a hard, brittle layer, known as the α-case, to be removed by pickling. The thickness of this layer is usually approx. 0.03 mm.

To counteract the unfavorable effect of the negative segregation with the precipitations in the inter-dendritic zones, according to the invention the castings, after the casting molds have been removed following the investment casting, are subjected to a heat treatment. This involves hot isostatic pressing (HIP) specifically at a temperature just below the β-transus temperature. It may be in the range from 710° C. to 760° C. and is preferably approximately 740° C. This causes the undesirable precipitations in the inter-dendritic zones to be dissolved again. There is no need for any preliminary age-hardening before or after the hot isostatic pressing. However, fine secondary phases precipitate again during the cooling following hot isostatic pressing, preferentially in the original inter-dendritic zones (cf. FIG. 3, 1000 times magnification). This leads to undesirable embrittlement of the material.

The objects have only a low ductility following the hot isostatic pressing.

To eliminate the disruptive precipitations, the castings are annealed in a chamber furnace under a shielding gas atmosphere (e.g. argon). A temperature range from approx. 780° C. to 860° C. with a duration of several hours, generally two hours, is selected for this purpose. In this context, there is a reciprocal relationship between the temperature and duration; at higher temperature, a shorter time is sufficient, and vice versa. Following the solution annealing, the castings are quenched with cold water. FIG. 4 (1000 times magnification) illustrates the microstructure following the solution annealing. Primary β-grains and, within the grains, very fine inter-dendritic precipitations (cf. cloud-like accumulation in the top left of the figure) can be seen. The objects which have been investment-cast using the process according to the invention have β-grains with a mean size of more than 0.3 mm in their crystal structure. This size is typical of the crystal structure achieved by the process according to the invention.

The mechanical properties achieved following the solution annealing are given in the table in FIG. 1.

It can be seen that the modulus of elasticity drops with an increasing temperature during the solution annealing, specifically as far as levels of 60,000 N/mm². The ductility values improve with decreasing strength and hardness. For example, after solution annealing for two hours at 800° C., a modulus of elasticity of 60,000 N/mm² combined with an elongation at break of approx. 40% and a fracture strength Rm of approx. 730 N/mm² are achieved. 

1. A process for casting objects from a β-titanium alloy comprising titanium-molybdenum with a molybdenum content of from 7.5 to 25%, which includes melting the alloy at a temperature of over 1770° C., investment-casting the molten alloy into a casting mold corresponding to the object to be produced, hot isostatic pressing, solution annealing and subsequent quenching.
 2. The process as claimed in claim 1, which includes using a cold-wall crucible vacuum induction installation for melting the β-titanium alloy.
 3. The process as claimed in claim 1 or 2, which includes carrying out the hot isostatic pressing at a temperature which is at most equal to a beta transus temperature of the titanium-molybdenum alloy and is no more than 100° C., preferably 40° C., below the beta transus temperature.
 4. The process as claimed in either of claims 1 and 2, which includes carrying out the solution annealing at a temperature of from approx. 700° C. to approx. 900° C.
 5. The process as claimed in claim 4, which includes carrying out the solution annealing at a temperature of from 780° C. to 880° C.
 6. The process as claimed in one of the preceding claims, which includes quenching in preferably cold, water following the solution annealing.
 7. The process as claimed in one of the preceding claims, which includes final hardening of the object.
 8. The process as claimed in claim 7, which includes carrying out the hardening at a temperature of from 600° C. to 700° C. 